Synchronous optical network (SONET) is a standard for optical telecommunications transport. It was formulated by the ECSA (European Speech Communication Association) for ANSI (the American National Standards Institute). The SONET standard is expected to provide the transport infrastructure for worldwide telecommunications for at least the next two or three decades.
The increased configuration flexibility and bandwidth availability of SONET provides significant advantages over the older telecommunications system. These advantages include the following:                Reduction in equipment requirements and an increase in network reliability.        Provision of overhead and payload bytes—the overhead bytes permit management of the payload bytes on an individual basis and facilitate centralized fault sectionalization.        Definition of a synchronous multiplexing format for carrying lower level digital signals and a synchronous structure that greatly simplifies the interface to digital switches, digital cross-connect switches, and add-drop multiplexers.        Availability of a set of generic standards that enable products from different vendors to be connected.        Definition of a flexible architecture capable of accommodating future applications, with a variety of transmission rates.        
In brief, SONET defines optical carrier (OC) levels and electrically equivalent synchronous transport signals (STSs) for the fiber-optic-based transmission hierarchy.
As stated above, SONET is a technology for carrying many signals of different capacities through a synchronous, flexible, optical hierarchy. This is accomplished by means of a byte-interleaved multiplexing scheme. Byte-interleaving simplifies multiplexing and offers end-to-end network management.
The first step in the SONET multiplexing process involves the generation of the lowest level or base signal. In SONET, this base signal is referred to as STS-1, which operates at 51.84 Mbps. Higher-level signals are integer multiples of STS-1, creating the family of STS-N signals. An STS-N signal is composed of N byte-interleaved STS-1 signals. For example, STS-3 is three times the rate of STS-1 (3×51.84=155.52 Mbps). An STS-12 rate would be 12×51.84=622.08 Mbps.
The frame 10 structure or format of the conventional STS-1 signal is shown schematically in FIG. 1. In general, the frame 10 can be divided into two main areas: transport overhead 12 and the synchronous payload envelope (SPE) 14.
The synchronous payload envelope 14 can also be divided into two parts: the STS path overhead (POH) 16 and the payload 18, as seen in FIG. 2. The payload 18 is the revenue-producing traffic being transported and routed over the SONET network. Once the payload is multiplexed into the synchronous payload envelope, it can be transported and switched through SONET without having to be examined, and possibly demultiplexed, at intermediate nodes. Thus, SONET is said to be service-independent or transparent.
The STS-1 SPE may begin anywhere in the STS-1 envelope capacity, as illustrated schematically in FIG. 2. Typically, it begins in one STS-1 frame and ends in the next. The STS payload pointer (which points to J1), contained in the transport overhead, designates the location of the byte where the STS-1 SPE begins.
STS POH is associated with each payload, and is used to communicate various information from the point where a payload is mapped into the STS-1 SPE to where it is delivered.
When the frame rate of the SPE is too slow in relation to the rate of the STS-1, certain bits of the pointer word (I-bits) are inverted in one frame, thus allowing 5-bit majority voting at the receiver. Periodically, when the SPE is about one byte off, these bits are inverted, indicating that positive stuffing must occur. This is illustrated schematically in FIG. 3. An additional byte is stuffed in, allowing the alignment of the container to slip back in time. This is known as positive justification or stuffing, and the stuff byte is made up of non-information bits. This is important due to the synchronous nature of SONET. The actual positive stuff byte immediately follows the H3 byte (that is, the stuff byte is within the SPE portion). The pointer is incremented by one in the next frame, and the subsequent pointers contain the new value. Simply put, if the SPE frame is traveling more slowly than the STS-1 frame, every now and then stuffing an extra byte in the flow gives the SPE a one-byte delay.
Conversely, when the frame rate of the SPE frame is too fast in relation to the rate of the STS-1 frame, bits 8, 10, 12, 14, and 16 of the pointer word are inverted, thus allowing 5-bit majority voting at the receiver. These bits are known as the D-bits or decrement bits. Periodically, when the SPE frame is about one byte off, these bits are inverted, indicating that negative stuffing must occur, as shown schematically in FIG. 4. Because the alignment of the container advances in time, the envelope capacity must be moved forward. Thus, actual data is written in the H3 byte, the negative stuff opportunity (within the overhead); this is known as negative justification or stuffing.
The pointer is decremented by one in the next frame, and the subsequent pointers contain the new value. Simply put, if the SPE frame is traveling more quickly than the STS-1 frame, every now and then pulling an extra byte from the flow and stuffing it into the overhead capacity (the H3 byte) gives the SPE a one-byte advance. In either case, there must be at least three frames in which the pointer remains constant before another stuffing operation (and therefore a pointer value change) can occur.
A SONET frame (STS-N or Vc (virtual concatenation)) can be specified using a so-called TelecomBus Interface. A conventional TelecomBus is standard in local TDM processing (within a single ADM) but cannot be transmitted over large distances. Thus, it is used at present to send TDM SONET signals a short distance between SONET cards in telecommunications equipment. One example of a conventional TelecomBus Interface is shown schematically in FIG. 5.
The TelecomBus consists of the following signals:                SPE-1 of data=payload, 0-otherwise        C1/J1-1 if data=c1 byte in section overhead or j1 byte in path overhead        Data—The corresponding data byte        
A SONET framer, which receives a SONET signal to be transported, is capable of producing the Telecombus from the SONET signal.
However, providing SONET services in current networks can be done only over dedicated SONET channels. This causes a great waste of bandwidth resources, which could have been shared between both SONET services and packet services. Another problem is difficult management of the SONET service trail. Each path has to be manually configured in any node it passes. Yet another difficulty is the synchronous nature of SONET—it is crucial to maintain synchronization, so as to be able to accurately reconstruct the data at the destination. This requires transportation of idle frames so as not to lose synchronization.
Accordingly, there is a long felt need for a method and system for providing both SONET services and packet services, and it would be desirable to have such a method which improves utilization of bandwidth resources.